In a PON (Passive Optical Network) system, an OLT (Optical Line Terminal: a station-side optical line terminal apparatus) and ONUs (Optical Network Units: user-side optical line terminal apparatuses) perform communication while synchronizing with each other to prevent data in an uplink direction transmitted from the ONUs from colliding with one another. The OLT forms a plan to give transmission permission to the ONUs to prevent the data in the uplink direction from colliding with one another. In forming the plan, the OLT takes into account delays due to distances between the OLT and the ONUs. Therefore, the OLT measures round trip times between the OLT and the ONUs. However, because there is fluctuation in a transmission path such as jitter or wander in transmission by an optical fiber, it is necessary to periodically perform the measurement.
On the other hand, data communication is not always performed. The data communication is not performed at all, for example, at night. However, the measurement of round trip times is periodically performed irrespective of presence or absence of the data communication as explained above. If an ONU is always set in a communicable state for the measurement of round trip times even when the data communication is not performed, electric power is wasted. Therefore, a technology is examined for intermittently transitioning the ONU to a power saving state by requesting, from the ONU, a shift to the power saving state.
A PON system is examined that does not allocate useless transmission bandwidths to an ONU when there is no uplink data from the ONU and improves throughput (Patent Literature 1). In the PON system, when an OLT detects a state in which there is no user data for a fixed period set in advance, the OLT deletes registration of the ONU and notifies the ONU that an optical link to the ONU is temporarily stopped. Thereafter, a transmission bandwidth is not allocated to the ONU and transmission of a frame for maintaining a link is suppressed. Therefore, the ONU can reduce the number of times of transmission of the frame.